1. Field of the Invention
The present invention relates to a carrier, a developer, a developing apparatus and an image forming apparatus.
2. Description of the Related Art
1Office automation equipments have been remarkably developed in these days and in line with such development, there has been a wide spread of copiers, printers, facsimile machines, and the like machines which form images through the electrophotographic system. In general, an image is formed by way of a charging step, an exposing step, a developing step, a transferring step, a cleaning step, and a fixing step in an image forming apparatus which employs the electrophotographic system. Specifically, at the charging step, a surface of a photoreceptor serving as an image bearing member is evenly charged in a dark place. At the exposing step, the charged photoreceptor receives signal light derived from a document image, resulting in removal of charges on the exposed part of the photoreceptor whose surface thus carries an electrostatic image (an electrostatic latent image). At the developing step, an electrostatic-image-developing toner (hereinafter simply referred to as “toner” unless otherwise mentioned) is supplied to the electrostatic image on the surface of the photoreceptor, thereby forming a toner image (a visualized image). At the transferring step, a recording medium such as paper or sheet is brought into contact with the toner image on the surface of the photoreceptor, and the corona discharge is then generated toward the recording medium from one side thereof which is reverse to the side in contact with the toner image, to thereby provide the recording medium with charges of which polarity is opposite to that of charges of the toner, thus transferring the toner image onto the recording medium. At the fixing step, the toner image on the recording medium is fixed by heat, pressure, or the like means. At the cleaning step, the toner is collected which has not been transferred onto the recording medium and thus remains on the surface of the photoreceptor. Through the above steps, a desired image is formed by the image forming apparatus employing the electrophotographic system.
A usable developer for developing a toner image in the image forming apparatus employing the electrophotographic system includes a one-component developer containing only a toner and a two-component developer containing toner and carrier. The two-component developer is characterized in forming a high-quality image more easily because functions of stirring, conveying, and charging toner particles can be given by the carrier. Accordingly, since toner in two-component developer does not need to have functions of carrier, the two-component developer has characteristics that the controllability is improved due to such separation of the functions, and a high-quality image is easily obtained, compared with one-component developer containing toner particles solely. Therefore, a lot of development and research have been conducted with respect to toner suitable for use in combination with carrier.
The carrier has two fundamental functions including a function of stably charging toner to a desired charge amount and a function of carrying toner to a photoreceptor. Further, the carrier is stirred in a developer tank and carried onto a magnet roller to form a magnetic ear, followed by passing through a regulation blade and returning to the inside of the developer tank again so as to be repeatedly used. While continuously used in this way, the carrier is required to stably exert the fundamental functions, particularly to stably charge toner. In order to maintain such a function of carrier, methods for coating the surface of the carrier with a resin have been proposed.
Examples thereof include one for coating the surface of carrier with a styrene-acrylic copolymer resin or a polyurethane resin, each of which has high surface tension, and one for coating with a silicone resin or a fluorine resin, each of which conversely has low surface tension. However, a resin having high surface tension has excellent adhesiveness to a carrier core, but has a problem that toner-spent is likely to occur. On the other hand, a fluorine resin having low surface tension is effective against the toner-spent, but due to poor adhesiveness to the carrier core, has a problem that the coating resin separates from the carrier core when the carrier is stirred in the developer tank, which inhibits stable charging.
In order to improve these problems, carrier coated with a resin obtained by hybridizing an acrylic resin and a silicone resin has been proposed.
For example, Japanese Unexamined Patent Publications JP-A 8-234501 (1996) and JP-A 2006-178508 disclose a carrier formed by coating a surface of a carrier core with a copolymer of a specific silicone macromonomer and an acrylic resin.
Further, For examples JP-A 2000-235283 discloses carrier having a coating layer consisting of an acryl-modified silicone resin.
Along with the advancement of full-color electrophotography in recent years, a lot of improvements have been made to toner. An improvement to a toner external additive is included partly. The toner external additive provides fluidity to toner and has a function as an auxiliary for controlling a charge amount.
In the full-color electrophotography, for the purpose of improving toner transfer efficiency, an external additive having a large particle size tends to be added. On the other hand, as a ratio of an external additive having a large particle size that is present on the toner surface is increased, chances of contact between the toner and the carrier are inhibited, which makes it difficult to charge the toner stably. Furthermore, color toner has higher insulating properties due to a material thereof than monochrome toner, which makes it difficult to stabilize the charging.
Against such problems of chargeability, since the carrier disclosed in JP-A 8-234501 and 2006-178508 uses a water-soluble resin, it is concerned that variations in the charge amount become large in accordance with environmental variations such as variations in temperature or humidity. Furthermore, since an acryl resin itself is uncrosslinked, rubbing resistance expected in actual use is low, therefore, stability of charging with respect to the number of print sheets cannot be expected that much.
Meanwhile, in the carrier disclosed in JP-A 2000-235283, an acryl-modified silicone resin and an aminosilane coupling agent are used for controlling a charge amount and compensating for a problematic low chargeability of a silicone with an acrylic resin.
However, it is difficult to control charging beyond charging capability of the acryl-modified silicone resin, and when trying to further increase the charge amount, it is necessary to add the aminosilane coupling agent for controlling. However, when the charge amount is increased by the aminosilane coupling agent, the charge amount is capable of being increased in an initial usage state, but is likely to be decreased as the number of print sheets is increased. That is, charging property of a coating resin itself greatly influences charging performance for the toner, and an increase in the charge amount of the coating resin itself is required.